What will happen if I don't wear my retainer for a week?

Typically patients are instructed to wear their retainers "regularly" for the first six months following active orthodontic treatment (braces/Invisalign/etc.) because this tends to be the period of time when the teeth are least stable in their new positions.  Each practitioner is different in terms of what is considered "regular".  

In our practice, we ask our patients to wear their retainers 12 hours/day for the first six months.  When they return at their six month check-in, if they report that their retainers fit well and comfortably, we then transition them to wearing their retainers only while they sleep.

Having said that, there is individual variability in all patient's response to treatment/retainers.  I would hope that following 5-7 days without the retainers in, they would still fit.  You can expect the retainers to be tight when you first replace them.  Hopefully, you can endure a few days of discomfort as the retainers recapture any shifting that may have occurred during the week.

If you cannot get the retainers back in, I would make an appointment to see your dentist/orthodontist to see if they can adjust the retainers so they fit properly.